In electronic devices, such as PDAs (personal digital assistants), laptop computers, office automation equipment, medical equipment, and car navigation systems, touchscreens are widely used as their display screens that also serve as input means.
There are a variety of touchscreens that utilize different position detection technologies, such as optical, ultrasonic, surface capacitive, projected capacitive, and resistive technologies. A resistive touchscreen has a configuration in which an optically transparent conductive material and a glass plate with a transparent conductive layer are separated by spacers and face each other. A current is applied to the optically transparent conductive material and the voltage of the glass plate with a transparent conductive layer is measured. In contrast, a capacitive touchscreen has, as its basic component, an optically transparent electrode in which an optically transparent conductive layer is provided on a base material, and does not have any movable parts. Due to high durability and high transmission, capacitive touchscreens are used in various applications. Further, projected capacitive technology allows simultaneous multipoint detection, and therefore is widely used for smartphones, tablet PCs, etc.
As optically transparent electrodes used for touchscreens, those having an optically transparent conductive layer made of ITO (indium tin oxide) formed on a base material have been commonly used. However, there has been a problem of low total light transmittance due to high refractive index and high surface light reflectivity of an optically transparent conductive layer made of ITO. Another problem is that an ITO conductive layer has low flexibility and thus is prone to crack when bent, resulting in an increased electrical resistance.
Known as an alternative to an optically transparent electrode having an optically transparent conductive layer made of ITO is an optically transparent electrode having a higher total light transmittance and a higher conductivity, the optically transparent electrode being obtainable by forming a mesh pattern of metal thin lines as an optically transparent conductive layer on an optically transparent base material, in which metal pattern, for example, the line width, pitch, pattern shape, etc. are appropriately adjusted. Regarding the pattern of the metal thin lines, it is known that a repetition unit of any shape may be used. For example, in Patent Literature 1, a triangle, such as an equilateral triangle, an isosceles triangle, and a right triangle; a quadrangle, such as a square, a rectangle, a lozenge, a parallelogram, and a trapezoid; a polygon, such as a hexagon, an octagon, a dodecagon, and an icosagon; a circle; an ellipse; and a star, and a combinational pattern of two or more thereof are used.
For the production of an optically transparent electrode using an optically transparent conductive layer consisting of metal thin lines, a semi-additive method comprising making a thin catalyst layer on a base material, making a resist pattern on the catalyst layer, making a laminated metal layer in an opening of the resist by plating, and finally removing the resist layer and the base metal protected by the resist layer for forming a conductive pattern is disclosed in, for example, Patent Literature 2, Patent Literature 3, etc.
Also, in recent years, a method in which a silver halide diffusion transfer process is employed using a silver halide photosensitive material as a precursor to a conductive material is known. For example, Patent Literature 4, Patent Literature 5, Patent Literature 6, etc. disclose a technology for forming a metal silver pattern by a pattern exposure and a reaction of a silver halide photosensitive material (a conductive material precursor) having a physical development nucleus layer and a silver halide emulsion layer in this order on a base material with a soluble silver halide forming agent and a reducing agent in an alkaline fluid. The patterning by this method can reproduce uniform line width. In addition, due to the highest conductivity of silver among all metals, a thinner line with a higher conductivity can be achieved as compared with other methods. An additional advantage is that an optically transparent conductive layer having a metal silver pattern obtained by this method has a higher flexibility, i.e., a longer flexing life as compared with an optically transparent conductive layer made of ITO.
Generally, in a projected capacitive touchscreen, an optically transparent electrode having two optically transparent conductive layers each having a sensor part formed of a plurality of column electrodes is used as a touch sensor. In such an application, a touchscreen in which a metal pattern having a repetition unit of any shape is used as a column electrodes has a problem. That is, an operator of the touchscreen usually keeps staring at the display, and as a result tends to recognize the metal pattern itself (the metal pattern is highly visible) . Also, in the optically transparent electrode having two optically transparent conductive layers overlapped with each other, depending on the shape of the metal pattern, moire can be caused, resulting in even higher visibility. Further, in the cases of an optically transparent electrode in which the metal pattern having a repetition unit is formed of very thin metal lines, the electrical resistance value can vary depending on the pattern shape under an atmosphere of high humidity and high temperature. There has been no known method to solve the above mentioned problem of moire and the problem on the stability of electrical resistance values at the same time.
To address these problems, Patent Literature 7 proposes a method in which column electrodes with a metal pattern of which the repetition unit is in the shape of a lozenge are used for one of two optically transparent conductive layers while column electrodes with a metal pattern of which the repetition unit is the same lozenge rotated 90° are used for the other optically transparent conductive layer. However, in this method, moire maybe seen depending on conditions, and the problem of unstable electrical resistance values under an atmosphere of high humidity and high temperature is not sufficiently solved.
For example, Patent Literature 8 etc. propose a method in which a diamond-like pattern is used as the metal pattern of column electrodes, and the upper and lower optically transparent conductive materials are superposed in such a manner that the metal patterns constituting the column electrodes of the two optically transparent conductive layers never overlap with each other for solving the problem of moire. However, in this method, the two optically transparent conductive layers need to be joined with very high positional accuracy, and insufficient accuracy tends to generate portions where the upper and lower patterns mistakenly overlap with each other or where no pattern exists, leading to even higher visibility. In addition, this method inevitably generates portions with narrower width of column electrodes, and such portions are more significantly affected by the problem of unstable electrical resistance values of the above-mentioned metal pattern having a repetition unit formed of very thin metal lines under an atmosphere of high humidity and high temperature.